A concentration measurement method that measures the concentration of a specific component in a biological object (subject) has been developed. For example, a technique has been developed that measures the blood glucose level by collecting blood from a fingertip or the like, and measuring the enzymatic activity against glucose in blood.
However, the above blood glucose level measurement method is invasive, and is painful or unpleasant for the subject. Therefore, a technique that applies near-infrared light to the surface of a part (e.g., hand) of a human body, and measures the blood glucose level from the light absorption has been developed as a non-invasive blood glucose level measurement method.
For example, JP-A-2010-237139, JP-A-2007-259967, and JP-A-2008-49091 disclose a technique that calculates the concentration of glucose in the dermis by utilizing reflected light (scattered light) obtained by applying light to the biological object.
JP-A-2007-259967 and JP-A-2008-49091 disclose a probe that is brought into contact with the biological object, and includes a plurality of irradiation sections that apply measurement light to the biological object, and one light-receiving section that receives scattered light from the biological object. In JP-A-2007-259967 and JP-A-2008-49091, the irradiation section and the light-receiving section are disposed so that the ratio of scattered light from the dermis increases. More specifically, scattered light from the dermis is selectively received by setting the distance between the irradiation section and the light-receiving section to a specific value (i.e., 0.65 mm).
When using the above method, however, light received by the light-receiving section may include light that has propagated in a skin layer (e.g., epidermis or subcutis) other than the dermis. The dermis contains a plurality of components other than glucose. The glucose concentration may not be accurately determined when light that has propagated in a skin layer other than the dermis (i.e., error) is mixed into light received by the light-receiving section. The above problem also occurs when the measurement target component is not glucose.